Charging two GP Ion-Li Battery packs with only one IC?

Question

Hello,

I have this two Li-Ion Battery packs from GP Batteries:

7.4V

http://es.farnell.com/jsp/search/productdetail.jsp?SKU=1848661

11.1V

http://es.farnell.com/jsp/search/productdetail.jsp?SKU=1848662

I have put them in series -> 18.5V

I want them to charge with for example LTC4008EGN#PBFLTC4008EGN#PBF from Linear Tech.

http://es.farnell.com/linear-technology/ltc4008egn-pbf/cargador-de-baterias-smd-ssop20/dp/1273983?Ntt=ltc4008

Question is, what happens if one of the packs is medium-charged and the other completely discharged? I understand that in a certain moment one of the packs will be charged, but what happens with the other pack?

Will continue charging? Will the charged pack be damaged? Or simply one of the packs will remain without being charged?

Is it imperative in this scenario to use one charger for each pack??

Thanks for any tip.

Regards,

Cristian.

RWM · Accepted Answer

Hi Cristian, as battery packs are equipped with protection circuits (over-charge over-discharge etc. - see datasheet page 6), they should be charged separately. If two packs will be connected in series and will be in different state of charge, when charging protection circuit in one pack will detect overvoltage, it will disconnect this pack, so charging of second pack will be stopped. If there will be other battery pack without protection circuits, it would be possible to connect packs in series and charge from one charger but blancing circuit should be used. It protects cells from overcharging.

RWM · Answer

Hi, as these packs have protection circuits which can disconnect pack, you have some options: 1) use 2 floating power sources (it means GND of both sources not connected, as GND will be connected to negative electrode of power pack), 2) remove protection circuits from power packs, connect them in series and use one charger; however, you should add protection circuit for 5 cells (for example http://www.ti.com/product/bq77pl157a4225 or more advanced http://es.farnell.com/texas-instruments/bq77pl900dl/ic-battery-protec-li-ion-10cell/dp/1640400 ). Regards, Ryszard

shoker4 · Answer

Hi Christian, by the characteristics of the battery I guess the use to R / C, told him that I had the same problem with Li-Ion batteries are quite sensitive with respect to the overtemperature and overvoltage. In addition, the battery pack that I use are of high capacity (5000mAh) and Vienne linked in series-parallel with no internal protection. I decided to break the pack and reset CI placed a pair of of protection against overload / over temperature and also balances the load of the cell. For the latter decided to take a pair of wires to each cell to the back of the pack and charger by using a load cell by cell in a battery, a load at the end of the cables so that the pack is reinforced parallel series. As an additional point of C.I. also use the download intelligently manage each cell preventing its voltage falls below 3.3V, which were to happen if the cell loses its ability to recharge.

Then I leave the link in the catalog in which the CI search more suitable to my application.

http://www.ti.com/lit/sg/slyt420/slyt420.pdf

Att. Gustavo (sorry my english is bad)

This is the original pack modifying:

__________________________________________________________________________________________________________________________________________

Hola cristian, por las caracteristicas de la bateria me imagino que la utilizarás para R/C, le comento que yo tuve el mismo inconveniente con las baterias Li-Ion, son bastante delicadas con respecto a la sobretemperatura y sobrevoltaje. Además los pack de bateria que utilizo son de alta capacidad (5000mah) y vienne unidas en serie-paralelo sin ninguna proteccion interna. Decidi deshacer el pack y rearmarlo colocandole un par de C.I. de proteccion contra sobrecarga/sobretemperatura y además balancea la carga de la celda. Para esto ultimo decidi sacar un par de cables a cada celda hacia el lado posterior del pack y utilizando un cargador mediante cargar celda por celda en un cargador, al finalizar la carga uno los cables de tal manera que queda armado el pack serie paralelo. Como punto adicional los C.I. que utilice tambien gestionan inteligentemente la descarga de cada celda evitando que su tensión descienda por debajo de 3.3v, que si llegara a pasar la celda pierde su capacidad de recargarse.

A continuación le dejo el link del catalogo en el cual busque los C.I. mas adecuados a mi aplicacion.

http://www.ti.com/lit/sg/slyt420/slyt420.pdf

Atte. Gustavo

Este es el pack original que modifique:

RWM · Answer

Hi Cristian, LTC4008 is constant current/constant voltage charger. This method is used mainly to charge various Lithium cells (Li-Ion, Li-Poly, LiFePO4) but can also be used to charge lead-acid batteries. For other chemistries is not recommended (NiCd and NiMH use mainly -dV method), but if you set float voltage as safety limit and add -dV detection, you can charge also Ni-based batteries. Termination of Li-Ion charging is when charging current decline to 0.05-0.3 of current in constant current mode. LTC4008 has FLAG output which is set low when charging current fall to C/10, so it can be used to e.g. shutdown charge using SHDN input. You can also connect FLAG to microcontroller which will shutdown charger usign SHDN input and also will monitor FAULT flag but this is not necessary.

RWM · Answer

Interesting document. Sometimes charging of deeply discharged battery can damage it, so it is better to implement low battery conditioning. Schematic is on figure 6 on page 14 in LTC4008 datasheet. You can drive Q1 using comparator/op-amp (when battery voltage is lower than 15V->3.0V/cell) or microcontroller if you will use it. I do hope your charger will work good. Best regards, Ryszard